In scanning microscopy, a specimen is illuminated with a light beam in order to observe the reflected or fluorescent light emitted by the specimen. The focus of an illuminating light beam is moved in a specimen plane by means of a controllable beam deflection device, generally by tilting two mirrors; the deflection axes are usually perpendicular to one another, so that one mirror deflects in the X direction and the other in the Y direction. Tilting of the mirrors is brought about, for example, by means of galvanometer positioning elements. The power level of the light coming from the specimen is measured as a function of the position of the scanning beam. The positioning elements are usually equipped with sensors to ascertain the present mirror position.
In confocal scanning microscopy specifically, a specimen is scanned in three dimensions with the focus of a light beam.
A confocal scanning microscope generally comprises a light source, a focusing optical system with which the light of the source is focused onto an aperture (called the “excitation pinhole”), a beam splitter, a beam deflection device for beam control, a microscope optical system, a detection pinhole, and the detectors for detecting the detected or fluorescent light. The illuminating light is coupled in via a beam splitter. The fluorescent or reflected light coming from the specimen travels through the beam deflection device back to the beam splitter, passes through it, and is then focused onto the detection pinhole behind which the detectors are located. Detected light that does not derive directly from the focus region takes a different light path and does not pass through the detection pinhole, so that a point datum is obtained which results, by sequential scanning of the specimen, in a three-dimensional image. A three-dimensional image is usually achieved by acquiring image data in layers, the track of the scanning light beam on or in the specimen ideally describing a meander (scanning one line in the X direction at a constant Y position, then stopping the X scan and slewing by Y displacement to the next line to be scanned, then scanning that line in the negative X direction at constant Y position, etc.).
For the examination of biological specimens, it is been usual for some time to prepare the specimen with optical markers, in particular with fluorescent dyes. Often, for example in the field of genetic investigations, several different fluorescent dyes are introduced into the specimen and become attached specifically to certain specimen constituents. From the fluorescence properties of the prepared specimen it is possible, for example, to draw conclusions as to the nature and composition of the specimen or the concentrations of particular substances within the specimen. In most cases, multiple lasers are used for simultaneous illumination with light of several wavelengths. EP 0 495 930, “Confocal microscope system for multicolor fluorescence,” discloses an arrangement having a single laser that emits several laser lines. In practice, mixed-gas lasers, in particular ArKr lasers, are generally used for this purpose. For detection, several detectors for detected light of different wavelengths are generally provided. One particularly flexible arrangement for simultaneous multicolor detection of detected light of several wavelengths is disclosed in German Patent DE 199 02 625, “Apparatus for simultaneous detection of several spectral regions of a laser beam.”
In addition to simultaneous multicolor detection, sequential detection of image data at different wavelengths also plays an important role in microscopy. Here the image data for the images at the different detected light wavelengths are obtained sequentially in time. The images can be displayed to the user in the form of several individual depictions, each individual depiction being associated with one detected light wavelength or one detected light wavelength region. Display of a superimposed depiction of the individual depictions is also usual; it is very important in this context that the image data of the individual depictions belonging to the same points in the specimen be precisely interassociated.
In the context of multicolor detection, the known microscopes have the disadvantage, because of chromatic aberrations of the optical system and especially because of longitudinal chromatic aberration, that at the different detection wavelengths, image data are unintentionally obtained from different specimen regions, e.g. from specimen section planes of different depths. This results in depictions or images that are not comparable, and is particularly disadvantageously evident in the superimposed depiction.
A scanning microscope that partly eliminates the aforementioned disadvantages is disclosed in German Unexamined Application DE 100 18 256 A1. The scanning microscope is characterized in that the optical properties in particular of the components arranged in the beam path are coordinated with one another in such a way that the cumulative aberrations are at least on the order of the theoretically achievable resolution with respect to the optical axis and/or at least one surface in the specimen region. This approach requires very complex and very expensive optics, and cannot be implemented simultaneously for the entire possible detection spectrum.